In order to scan a subsea structure, such as a pipeline, it is known to mount an apparatus on the structure and then to rotate at least a part of the apparatus around the structure. An example of such an apparatus is described in GB 2496736 A. That apparatus comprises a source of gamma radiation and an array of detectors spaced apart circumferentially. The apparatus is capable of being arranged with the structure to be scanned, such as a pipeline, positioned between the source and detectors so that radiation emitted by the source can pass along the plurality of paths through a portion of the structure to the detectors. Counting the number of gamma photons transmitted from the source to the detectors, through the structure being scanned, enables differences in the density of different parts of the structure to be detected. The detectors are arranged in an arc centred on the structure to be scanned. In operation, the source and array of detectors are arranged in fixed relationship with respect to each other, and are rotated around the structure to be scanned. In this way, information about the density of the structure along a plurality of paths is obtained, enabling a high resolution density tomogram of the structure to be calculated. The apparatus can then be moved to a new position on the structure and a new scan completed.
The detectors, which are mounted on a rotating part of the apparatus, require a source of power in order to operate. It is desirable that the apparatus can be rotated continuously in one direction. Stopping the apparatus and reversing the direction of rotation takes time and may increase wear on the apparatus. Thus the source of power is desirably located on the rotating part so that no cable connections are required to the rotating part. That source of power can be a battery or other power storage device, which can be charged before deployment of the apparatus. However, a battery has limited capacity and must be recharged periodically. That has to date been done in one of two ways: the apparatus can be returned to the surface and a cable plugged into a port on the rotating part to charge the battery, or the apparatus can remain in position subsea with rotation of the apparatus halted and a Remotely Operated Vehicle (ROV) can connect a cable to a stab port on the, now stationary, rotating part and the battery can be recharged subsea via the ROV. Both of those techniques require the apparatus to cease operation, which may significantly increase the time taken to scan a lengthy structure such as a pipeline. There exists therefore a need to improve the powering of electronic devices on a rotating part of a subsea apparatus.
When conducting scans, and particularly when constructing tomograms, it is desirable to have highly accurate information about the rotational positions at which each part of the scan was completed. There is thus a need for improved systems for recording the rotational position of rotating parts of subsea apparatus.
Typically a scanning apparatus would be lowered onto the subsea structure to be scanned from above. However, during the rotating scan process parts of the apparatus may pass underneath the structure. If there were to be a failure of the drive mechanism when part of the apparatus is underneath the structure, it may not be possible to lift the apparatus off the structure. In such circumstances, it may be necessary to use an ROV to grab the rotating part of the apparatus and then attempt to rotate it using the ROV. It will be appreciated that such a manoeuvre may be challenging to perform.
In subsea applications, additional constraints arise. When operating at a depth of 1000 meter underwater, the pressure is 100 atmospheres and increases by a further 100 atmospheres for each additional 1000 meter of depth. The apparatus must be able to withstand this pressure yet remain sufficiently compact for deployment using remotely operated vehicles capable of operating at the required depth. Any increase in size of the apparatus can also result in significant increases in the force to which the apparatus is subject, since the high pressure of the water is then applied across a larger surface area.
Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art.